1. Field of the Invention
This invention relates to an exposure control device for use in a camera. More particularly, the invention concerns improvements in an exposure control device for opening and closing iris diaphragms in automatic cameras such as a video camera, which has an automatic exposing function capable of automatically determining the correct exposure.
2. Description of the Prior Art
Among automatic cameras such as video cameras, there are cameras equipped with an automatic exposure controlling means for automatically selecting and setting the iris aperture (exposure) formed by a plurality of iris diaphragms in accordance with the brightness of a subject to be taken as an image. There have heretofore been proposed some which have an exposure controlling mechanism being simple in construction to meet requirements for a small and easily portable automatic video camera capable of stably setting the iris aperture.
As proposed by Japanese Utility Model Application Public Disclosure Nos. SHO 59-4524(A); SHO 62-6711(A); SHO 62-9235(A); SHO 62-84031(A); and SHO 62-91625(A), conventional cameras are provided with a pair of iris diaphragms which move slidably in opposite directions so as to adjust the iris aperture defined by exposure spaces formed in the respective iris diaphragms. Each of the iris diaphragms used in the conventional cameras has two parallel slots in its side portions and is movably supported by guide pins which are planted on a base plate, as proposed in Japanese U.M. Appln. Pub. Discl. No. SHO 59-4524(A). The iris diaphragms are driven to move parallel in opposite directions with rocking motion of a diaphragm driving lever attached to a motor, so that the iris aperture can be adjusted.
In the other prior art cameras (for example, Japanese U.M. Appln. Pub. Discl. No. SHO 62-84031(A)), as illustrated in FIG. 1(A), a pair of iris diaphragms r1, r2 each having a guide hole h are engaged with the respective driving pins p planted on either end portion of a diaphragm driving arm a which is rockingly moved by a motor m. By operating the motor m to move parallel the iris diaphragms r1, r2 in opposite directions, the iris aperture is adjusted to change the intensity of the light passing through the iris aperture for exposure in accordance with the brightness of a subject to be taken as an image. The iris diaphragms r1, r2 are slidably supported between side guide walls w formed on a base plate in a state being in line contact with the side guide walls w, so that the diaphragms can move parallel.
The iris diaphragm is generally made of a thin plate of metal plate or synthetic resin of about 50 .mu.m to 150 .mu.m thickness and can be expected to move smoothly and permit fast response. To efficiently move such thin iris diaphragms with slight torque, driving loss should be reduced to the fullest possible extent. However, two iris diaphragms r1, r2 in the prior art exposing mechanism are in frictional contact with the opposite side guide walls w (as shown in Japanese U.M. Appln. Pub. Discl. No. SHO 62-84031(A), for example) or guide pins which are engaged slidably with guide slots formed in the iris diaphragms (as in Japanese U.M. Appln. Pub. Discl. No. SHO 59-4542(A)), with the result that frictional resistance produced between the diaphragms and the guide means such as the guide walls or guide slots is increased. A low power driving motor incorporated within a compact camera is adversely affected by the frictional resistance. Though the frictional resistance between the iris diaphragms and the guide means can be reduced by providing sufficient clearance therebetween, wobbling of the iris diaphragms is however caused due to the clearance, thereby involving a decrease in exposure accuracy.
Besides, there is a case where the kinetic efficiency of the iris diaphragms is decreased when two pairs of the guide means for slidably supporting the iris diaphragms are not precisely positioned. Thus, the iris diaphragms are generally set with sufficient clearance in relation to the guide means in consideration of an error of position of the guide means, thereby to involve wobbling the iris diaphragms and decrease the stability of motion of the iris diaphragms as a matter of course.
On the other hand, the prior art exposure control device as proposed by Japanese U.M. Appln. Pub. Discl. No. SHO 62-84031(A) has iris diaphragms r1, r2 which move in one direction (direction d) substantially orthogonal to the lengthwise direction of an iris driving arm a as shown in FIG. 1(B). In this device, however, when the iris driving arm a assumes its extreme angular position as illustrated, there is produced a difference .DELTA.V in motion vector between the motion vector Va of the arm a and the motion vector Vr in the direction in which the iris diaphragms r1, r2 are allowed to move. The difference .DELTA.V in motion vector comes to a kinetic loss. The same is true of Japanese U.M. Appln. Pub. Discl. No. SHO 59-4524(A).
Furthermore, another camera shutter disclosed in U.S. Pat. No. 3,938,168 has a pair of iris diaphragms which are slidably supported by guide grooves formed in the side edge portions of a casing. This shutter mechanism has entailed a disadvantage that frictional resistance produced between the iris diaphragms and the guide grooves is increased when the iris diaphragms are firmly supported in order to be prevent wobbling, and otherwise, the iris diaphragms would move unstably with wobbling when they are loosely supported.